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SQL ALTER statement does not allow to alter a column fron NOT NULL to NULLs allowed. You need to frop the table and re-create and, obviously, before DROPPING it, you need to save its data.
You are allowed (without DROpping it) to alter a column from NULLs allowed to NOT NULL, but there are some restrictions and you need anyway to reorg the table before you could access it again.

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